Trauma, pathological degeneration, or congenital deformity of tissues may result in the need for surgical reconstruction or replacement. Reconstructive surgery is based upon the principle of replacing these types of defective tissues with viable, functioning alternatives. In skeletal applications, surgeons have historically used bone grafts. The two main types of bone grafts currently used are autografts and allografts. An autograft is a section of bone taken from the patient's own body, while an allograft is taken from a cadaver. This method of grafting provides the defect site with structural stability and natural osteogenic behavior. However, both types of grafts are limited by certain uncontrollable factors. For autografts, the key limitation is donor site morbidity where the remaining tissue at the harvest site is damaged by removal of the graft. Other considerations include the limited amount of bone available for harvesting, and unpredictable resorption characteristics of the graft. The main limitation of allografts has been the immunologic response to the foreign tissue of the graft. The tissue is often rejected by the body and is subject to the inflammatory response. Allografts are also capable of transmitting disease. Although a thorough screening process eliminates most of the disease carrying tissue, this method is not 100% effective.
Conventional orthopedic implants such as screws, plates, pins and rods serve as loadbearing replacements for damaged bone and are usually composed of a metal or alloy. Although these implants are capable of providing rigid fixation and stabilization of the bone, they cause improper bone remodeling of the implant site due to the large difference in the modulus between bone and metal.
These limitations have initiated the search for a dependable synthetic bone graft substitute. However, in order for an implant to be used as a replacement for bone, it must be capable of both osteointegration and osteoconduction. Osteointegration refers to direct chemical bonding of a biomaterial to the surface of bone without an intervening layer of fibrous tissue. This bonding is referred to as the implant-bone interface. A primary problem with skeletal implants is mobility. Motion of the implant not only limits its function, but also predisposes the implant site to infection and bone resorption. With a strong implant-bone interface, however, mobility is eliminated, thus allowing for proper healing to occur. Osteoconduction refers to the ability of a biomaterial to sustain cell growth and proliferation over its surface while maintaining the cellular phenotype. For osteoblasts, the phenotype includes mineralization, collagen production, and protein synthesis. Normal osteoblast function is particularly important for porous implants that require bone ingrowth for proper strength and adequate surface area for bone bonding. In addition, implants should be both biocompatible and biodegradable.
Calcium phosphate-based materials have been widely investigated for use as bone replacement materials. Most calcium phosphate biomaterials are polycrystalline ceramics characterized by a high biocompatibility, the ability to undergo osteointegration, and varying degrees of resorbability. Implants made from these materials can be in either a porous or non-porous form. Examples of commercially available calcium phosphate materials include Interpore 200.RTM. and Interpore 500.RTM.. Surgical models using porous calcium phosphate-based implant materials, however, have shown that porous implants heal more slowly than both autografts and empty defects (Nery et al. J. Periodotol. 1975 46:328; Levin et al. J. Biomed. Mat. Res. 1975 9:183). Studies on tissue ingrowth in non-resorbable implants have also shown that failure of tissue to completely fill the implant can lead to infection (Feenstra, L. and De Groot, K. "Medical use of calcium phosphate ceramics" In Bioceramics of Calcium Phosphate, De Groot, K. Ed., CRC Press, Boca Raton, Fla., 1983, pp 131-141; Feldman, D. and Esteridge, T. Transactions 2nd World Congress Biomaterials Society, 10th Annual Meeting, 1984, p 37.
Implants synthesized from the calcium phosphate-based material, hydroxyapatite (HA), the major mineral constituent of bone, are commercially available in a porous and non-porous form. Synthetic HA implants have excellent biocompatibility. Blocks of dense HA are not useful in reconstructive surgery because they are difficult to shape and do not permit tissue ingrowth. However, in a non-porous, particulate form, HA has been used successfully in both composite (Collagraft.RTM.) and cement (Hapset.RTM.) forms (Chow et al. Mater. Res. Soc. Symp. Proc. 1993 179:3-24; Cornell, C. N. Tech. Orthop. 1992 7:55).Due to its fragility and lack of compliance, porous HA have been largely limited to dental and maxillofacial surgery.
Tricalcium phosphate (TCP) is the other main type of calcium-phosphate based implant material. It has a biocompatibility similar to HA, but it is more resorbable than HA due to its crystal structure. The chemical structure of TCP allows it to be used as a calcium phosphate cement (CPC) (Chow, L. C. Centen. Mem. Issue Ceramics Soc. Jpn 1991 99:954; Mirtchi et al. Biomaterials 1989 10:475) which can be mixed in the operating room and thus can be easily molded to fit the implantation site. The compliance of TCP materials allow them to be used in a broader range of surgical applications than conventional ceramics.
Other types of ceramic bone replacement materials are based on silicate. The use of silicate based materials in bone replacement is associated with its biocompatibility. Unlike calcium-phosphate materials, silicate does not exist naturally in the body. However, its biocompatibility is similar to naturally occurring minerals. Examples of silicate based bone replacements include bioactive glasses.
Glass-ionomers, composite biomaterials containing both organic and inorganic components, have also been suggested for use in bone replacement.
However, a major disadvantage of many of the orthopaedic materials in current use is their lack of flexibility and inability to be custom fit to the implant site. Synthetic bone grafts come in a manufactured form that forces the surgeon to fit the surgical site around the implant. This can lead to increases in bone loss, trauma to the surrounding tissue and delayed healing time. By using an implant that can be shaped to the implant size, a customized fit can be obtained. This capability allows an implant to be used universally in all patients.
Polymers are a class of synthetic materials characterized by their high versatility. The versatility has led to the development of biodegradable, biocompatible polymers created primarily for use in medical applications. One of the most common polymers used as a biomaterial has been the polyester copolymer poly(lactic acid-glycolic acid) referred to herein as PLAGA. PLAGA is highly biocompatible, degrades into harmless monomer units and has a wide range of mechanical properties making this copolymer and its homopolymer derivatives, PLA and PGA, useful in skeletal repair and regeneration (Coombes, A. D. and Heckman, J. D. Biomaterials 1992 13:217-224; Mikos et al. Polymer 1994 35:1068-1077; Robinson et al. Otolaryngol. Head and Neck Surg. 1995 112:707-713; Thomson et al. J. Biomater. Sci. Polymer Edn. 1995 7:23-38; Devin et al. J. Biomateri. Sci. Polymer Edn. 1996 7:661-669).
Porous, three-dimensional matrices comprising these polymers for use in bone replacement have been prepared using various techniques. Coombes and Heckman (Biomaterials 1992 3:217-224) describe a process for preparing a microporous polymer matrix containing 50:50 PLAGA:PLA and 25:75 PLA:PLAGA. The polymer is dissolved in poor solvent with heat and the gel is formed in a mold as the polymer cools. Removal of the solvent from the matrix creates a microporous structure. However, the actual pore size of this matrix (&lt;2 .mu.m) is inadequate for bone ingrowth which requires a pore size falling within the range of 100-250 .mu.m for cell growth to occur. Further, the gel cast material undergoes a significant reduction in size (5-40%) due to the removal of the solvent thus leading to problems in the production of specific shapes for clinical use. Since the amount of shrinkage varies from sample to sample, changing the mold size to compensate for the shrinkage will not result in a consistent implant size.
Robinson et al. (Otolaryngol. Head and Neck Surg. 1995 112:707-713) disclose a sintering technique to produce a macroporous implant wherein bulk D,L-PLA is granulated, microsieved, and sintered slightly above the glass transition temperature of PLA (58.degree.-60.degree. C.). Sintering causes the adjacent PLA particles to bind at their contact point producing irregularly shaped pores ranging in size from 100-300 .mu.m. While the implants were shown to be osteoconductive in vivo, degradation of PLA caused an unexpected giant cell reaction.
Particulate leaching methods, wherein void forming particles are used to create pores in a polymer matrix have been described by Mikos et al., Polymer 1994 35:1068-1077 and Thomson et al., J. Biomater. Sci. Polymer Edn 1995 7:23-38. These methods produce highly porous, biodegradable polymer foams for use as cellular scaffolds during natural tissue replacement (Mikos et al., Polymer 1994 35:1068-1077; Thomson et al., J. Biomater. Sci. Polymer Edn 1995 7:23-38). The matrices are formed by dissolving PLA in a solvent followed by the addition of salt particles or gelatin microspheres. The composite is molded and the solvent allowed to evaporate. The resulting disks were then heated slightly beyond the T.sub.g for PLA (58.degree.-60.degree. C.) to ensure complete bonding of the PLA casing. Once cooled, the salt or gelatin spheres are leached out to provide a porous matrix. However, in both types of particulate leaching methods, the modulus of the matrix is significantly decreased by the high porosity. Thus, while these matrices might perform well as cellular scaffolds, in other applications such as bone replacement, their low compressive modulus would result in implant fracture and stress overloading of the newly formed bone. These problems could further lead to fractures in the surrounding bone and complete failure at the implantation site.
Laurencin et al. described a salt leaching/microsphere technique to induce pores into a 50:50 PLAGA/HA matrix (Devin et al. J. Biomater. Sci. Polymer Edn 1996 7:661-669). In this method, an interconnected porous network is made by the imperfect packing of polymer microspheres. The porous matrix is composed of PLAGA microspheres with particulate NaCl and HA. The particulate NaCl is used to widen the channels between the polymer microspheres. The hydroxyapatite is used to provide added support to the matrix and to allow for osteointegration. In this method, PLAGA is dissolved in a solvent to create a highly viscous solution. A 1% solution of poly(vinyl alcohol) is then added to form a water/oil (w/o) emulsion. Particulate NaCl and HA are added to the emulsion and the resulting composite mixture is molded, dried, and subjected to a salt leaching step in water. The resulting matrix is then vacuum dried, and stored in a desiccator until further use.
In vitro studies by Laurencin et al. showed osteoblast attachment and proliferation to the three-dimensional porous matrix produced by this salt leaching/microsphere method (Attawia et al., J. Biomed. Mater. Res. 1995 29:843-848; Attawia et al., Biochem. and Biophys. Res. Commun. 1995 213:639-644). Further, osteoblasts were shown to maintain their phenotype as demonstrated by the secretion of osteocalcin and alkaline phosphatase. Thus, the PLAGA/HA matrix fulfills the osteoconductive and osteointegrative requirements of a bone graft replacement. In addition, all components of the matrix are biocompatible, and the presence of ceramic HA particles allows for direct bonding to bone. However, during degradation in vitro, the mechanical strength of this matrix decreased to the lower limits of trabecular bone. Accordingly, in vivo implantation of this matrix could result in the mechanical failure of the implant or stress overloading of the newly regenerated osteoblasts.
In the present invention, a biodegradable, biocompatible polymer/ceramic composite composed of a poly(lactic acidglycolic acid) PLAGA! and hydroxyapatite HA! for use in three dimensional constructs for tissue engineering was prepared by three novel methods: the sintered microsphere method, the solvent casting method, and the gel microsphere method. Three-dimensional matrices produced by these methods are porous and act as cellular scaffolds during bone regeneration.